1. Field of the Invention
The present invention relates to an automatic transmission, in particular, relates to the flow of oil between sumps in different sections of the transmission.
2. The Prior Art
An automatic transmission for a four-wheel drive vehicle, includes a transmission, a transfer mechanism for distributing an output torque to front and rear wheels, and a chain transmitting mechanism.
When an automatic transmission such as the one described above, is installed in the usual off-road vehicle, the automatic transmission includes a transfer mechanism with a chain transmitting mechanism provided in a case independent of the transmission oil circuit and is supplied oil from an individual oil pump, because there was space for installation.
Recently, it has become necessary to make a transmission structure in more compact size and lighter weight for installing in a vehicle in a restricted space. In the case of the independent transfer mechanism, the temperature of the oil is increased by stirring it in the chain housing whereby the oil is degraded, the viscosity of the oil is decreased, and the cooling efficiency of the oil is decreased. As the result of this the efficiency of the rotating elements such as friction members or chain links is decreased, causing failure. Under the above circumstance, it is preferred that the transfer mechanism and the transmission have unitary construction with the chain transmitting mechanism oil circuit.
However, the oil in the transmission is forced to one end by inertia, the decrease of fluidity, and the inclination during fast starting, low temperature and acceleration when the transfer mechanism is integrated into the transmission. Under these circumstances, since the oil level in the oil pan disposed in the lower portion of the transmission is inclined to the front end or is low, the oil level can become lower than the suction port, in which case the oil supply to the oil pump is ceased by suctioning air and the oil fluid cannot be supplied to each member of the automatic transmission. Therefore, it becomes impossible to control the transmission and it may be impossible to start the vehicle. In order to prevent the above phenomenon, it is proposed that the oil pan be located lower to prevent the oil from flowing to the rear end during automatic rear inclination and to keep sufficient oil in the oil pan. However, it is very difficult to provide such a construction as above described for installation in a vehicle, except for an off-road vehicle.